1. Field of the Invention
The present invention relates to a plastic, flexible window shield, particularly a foldable rear window shield for a car top or hood. It also relates to a process for producing such a flexible window shield.
2. Discussion of the Background
Numerous different demands are made on flexible, plastic window shields. On the one hand they must have good optical characteristics, so as to make it possible to look through them in a distortion-free manner and must be adequately rigid so that in the taut state, i.e. with the top closed, they are as free from undulations as possible. On the other hand they must be sufficiently flexible to be able to participate in the folding process, but without bending. They must also have a high mechanical strength, a good scratch resistance and a high UV stability. The required characteristics must also be maintained over a long period of time, as well as at high and low temperatures.
The known flexible window shields comprise multilayer sheets. Thus, e.g. EP 85 006 B1 discloses a flexible window shield, which is made from an impact-resistant and tear-resistant core layer of polycarbonate or polyurethane-polyurea, on which are placed cover layers of highly elastic polyurethane with self-healing characteristics. The polyurethane-polyurea forming the core layer has in said known flexible window shield a substantially linear molecular structure and aliphatically or cycloaliphatically bound urethane and urea segments and is obtained by the reaction of a free isocyanate group-containing prepolymer with primary, aliphatic or cycloaliphatic diamines having an average molecular weight of 60 to 340, in which the prepolymer is produced from a polyester diol or a polyether diol with in each case an average molecular weight of 400 to 4000, or from a mixture of such polyester diols and polyether diols and shortchain, aliphatic diols with an average molecular weight of 62 to 250 and an aliphatic or cycloaliphatic diisocyanate, which is present in excess compared with the free hydroxyl groups. Although such a polyurethane has the necessary rigidity and tear resistance, but not the desired scratch resistance, onto said core layer are applied scratch-proof cover layers made from a relatively flexible, highly elastic polyurethane. As a result of its multilayer structure the production process for said known, flexible window shields is relatively complicated and costly. A flexible window shield known from DE 36 36 264 has a multilayer construction. In this case a transparent, flexible PVC sheet is provided on at least one side with a covering layer of an elastic, transparent two-component polyurethane varnish based on hydroxyl group-containing polyesters or hydroxyl group-containing polyacrylates or mixtures therefrom, which are crosslinked with polyisocyanates.
The problem of the invention is to provide a flexible window shield having a simple structure, which is able to fulfill the requirements regarding the mechanical strength characteristics and also have the necessary characteristics with respect to the scratch resistance.